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Water Level Sight Tube - completely DARK, Why? ...drainback tank


New Member
Nov 20, 2021
I am looking at a used AET drainback tank, 10 Gal, with a heat exchanger coil inside. The level sight tube is completely dark - so dark, I thought it was broken and replaced with a dark plastic tube. Seller says it's still glass and is "sediment" discoloration.

Anyone seen something like this? Possible cause?


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Not uncommon in sight glasses. You should be able to pull the fittings off, get the tube out, then a thin bottle brush & soapy water should clean that right up.
Thanks Rednecktek!

My main concern is whether the dark deposit could be residue from melted/degraded PEX or acidified glycol.

If these tanks are known for this (dark deposits on the sight glass) in otherwise healthy systems, then cleaning should be relatively easy.

I appreciate you sharing your experience!

Redish deposits - often Iron
Black deposits - often manganese
remove and clean with chlorine bleach and a brush.
does this tank have an anode? I would check it's condition, see if needs replacement.
...good idea on the anode. I'll check once I have it in my shop.

I am encouraged by that the deposits are not more alarming than a cleaning issue. Thanks for the comment!
Tanks collect debris, it is a good idea to flush the tank a once or twice a year to a drain, to clear any debris before it accumulates too much.
You don't need to dump the entire volume, just flush from the drain valve until it runs clear.
I do my tanks on my B-Day each year, that is my reminder.

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